Boris Johnson's office today defended comments in which the mayor criticised the failure of "common sense policing" in the handling of the Jean Charles de Menezes case, in a behind-the-scenes row with Sir Ian Blair, the Metropolitan police chief he later forced out.

An exchange of letters between the two men last year, obtained by Channel 4 News, reveal how Johnson infuriated Blair by his refusal to withdraw the claim that the police involved in the shooting of De Menezes had been "too trigger happy".

Asked by Blair to retract his "outrageous" comments, made during the mayoral campaign last year, Johnson refused, claiming that "common sense policing" would have avoided the "catastrophe" which saw an innocent man shot dead.

A spokeswoman for Johnson said that the London mayor, who is now chair of the Metropolitan Police Authority, believed that many others shared the concerns he raised in the letters.

"The mayor expressed a considered view as a commentator and mayoral candidate. The letter makes it clear that he is not questioning the courage of the officers at the scene. He stands by the concerns he raised and believes they are shared by many in London and across the UK," she said.

The letters, published last night using freedom of information legislation, show the row began in November 2007, when Blair wrote to the then-mayoral candidate after being incensed by his public suggestion that armed officers who mistook De Menezes for a terrorist and shot him dead could have been "too trigger happy".

The letters showed Blair demanded a retraction of the comments, but was strongly rebuffed. "I consider your comments that it could be argued that MPS [Metropolitan Police Service] officers are 'trigger happy' to be outrageous. I would remind you that out of over 10,000 potential firearms incidents attended by the MPS in the past year, shots were fired on only three occasions," he wrote to Johnson.

"I would suggest in the strongest possible terms that you withdraw your remarks."

An unrepentant Johnson refused. "I have absolutely no intention of doing so," he wrote. "It is hard to think of any other description of a catastrophe in which a completely innocent man ends up with seven bullets in his head."

While the officers involved had been "personally extremely brave", he said, no one had answered the question of "why the hell" a suspected suicide bomber was allowed to take two buses and then get on an underground train before being shot dead. "Why was he allowed to put the public at progressively greater risk? Many people believe that common sense policing would have allowed his identity to be established at a much earlier stage."

In a final response, Blair said he was "very, very surprised" Johnson would not take back the comment, which had led to many complaints from armed officers.

The claim was "offensive to the men and women who routinely have to face some of the most dangerous situations within policing, to protect the Londoners they serve and their unarmed colleagues", he said.

"These courageous men and women volunteer: they accept that they will have to stand between armed criminals and the public in life and death situations, which demand split second decision-making."

In advance of the inquest into the death, "those without knowledge of what they will say should think long and hard before making comment", he suggested.

Last month a jury returned an open verdict at the inquest into the death of 27-year-old De Menezes, whom police mistook for would-be terrorist Hussain Osman the day after four botched attacks on the London public transport system.

In giving their verdict jury members rejected police accounts of the shooting and the Scotland Yard argument that the killing was lawful.

Johnson was held responsible for the departure of Blair, who stepped down last October after the mayor used his role as chair of the MPA to make clear he had no confidence in him.

A replacement for the Met commissioner is expected to be announced soon.

The final interviews were held yesterday by Jacqui Smith, the home secretary, and Johnson.

The two candidates for the top job are Sir Hugh Orde, and Sir Paul Stephenson.

Orde, 50, is chief constable of the Northern Ireland Police Service and Stephenson is acting commissioner of the Met.